1. Field of the Invention
This invention relates to a nonwoven fibrous material and, more particularly, to a nonwoven composite insulation material that includes a layer of inorganic fibers bonded together by a carrier web of blended organic and inorganic fibers.
2. Description of the Prior Art
It is well known in the art of thermal and sound insulation to bond together glass fibers in nonwoven, felt-like layers by a resinous binder. The binder may be of either the thermosetting or the thermoplastic type. Examples of this type of insulation material are disclosed in U.S. Pat. Nos. 2,579,035; 2,598,102; 2,612,162; 2,633,433; and 3,144,376. Mineral wool, also identified as rock wool, slag wool, or mineral cotton is a loose fibrous material also known for thermal and sound insulation properties. In addition, mineral wool is used to fabricate synthetic resin-bonded panels for specific structural purposes and has application as a filtering medium and a fire-proofing material.
Mineral wool is an inorganic material in the form of a mass of finely intertwined fibers formed by blowing air or steam through molten rock or slag. A three-dimensional layer of intertwined mineral wool fibers lacks structural integrity since the fibers, even though intertwined, are brittle. Thus, without treatment, a layer of mineral wool fibers lacks structural strength for handling and installation as an insulator, a filter medium, a fireproofing material, etc. To overcome this deficiency, the fibers of a layer of mineral wool must be bonded together so as to resist splitting and delamination of the material when placed in use. Furthermore, because mineral wool is substantially a coarse and abrasive material, special handling procedures are required to permit efficiency in fabrication and use.
It is a known practice to bond together the fibers of a mineral wool layer by the use of resin, as is generally disclosed in U.S. Pat. No. 3,778,334. A thermosetting resin is applied to mineral wool fibers as they are spun to form a mass of intertwined mineral wool fibers. The resin binds the individual fibers together to prevent delamination of the layer. The resin is generally applied in the form of an aqueous solution, such as a water insoluble thermosetting resin in liquid form, an aqueous dispersion of a water insoluble thermosetting resin, or in a dry, powdered finely divided form.
The use of thermosetting resins as binders for mineral wool, or for any fibrous material in general, to form a nonwoven fibrous structure is objectionable because of the health hazard presented during the application of the resin to the fibrous layer. It is a common practice to disperse the resin in both a powder and liquid spray form which causes the resin to circulate into the air presenting an unhealthy working environment. Also, resin in liquid form must be carefully handled so as to prevent contamination of a public water system. For these reasons it is preferred to avoid the use of a resinous binder to form nonwoven insulation materials from inorganic fibers, such as rock wool.
Various alternatives to resin bonding of inorganic fibers to form a nonwoven, felt-like material are disclosed in U.S. Pat. Nos. 2,908,064; 3,317,335; 3,338,777; 3,608,166; 3,616,031; 3,917,448; 3,975,565; 4,081,582; and 4,237,180. U.S. Pat. No. 3,917,448 discloses forming a nonwoven material including a percentage of heat-shrinkable synthetic fibers blended with non-shrinkable fibers into a web in which both the shrinkable and non-shrinkable fibers are randomly arranged in three dimensions. These fibers are so entangled that when exposed to heat treatment the shrinkable fibers contract to mechanically interlock the shrinkable and non-shrinkable fibers and provide the web with a preselected thickness and density.
Similarly, U.S. Pat. No. 4,237,180 discloses an insulation material formed by a blend of organic and inorganic fibers processed by carding or garnetting to form a composite insulating fibrous material or a preselected thickness. About two to ten percent by weight of heat sensitive organic fibers, such as polyester fibers, are oriented within the composite material by a needling process to interlock with the inorganic fibers and compress the composite material to the preseleted thickness. The interlocking arrangement of organic and inorganic fibers are subjected to a shrinking treatment in which the organic fibers contract and bind the inorganic fibers together to form a composite insulating material having a tensile strength sufficient to prevent splitting and delamination of the composite body. However, an essential step in the formation of these types of nonwoven fibrous insulation materials is heat treating the blend of organic and inorganic fibers to shrink the organic fibers so as to bond together the inorganic fibers.
On the other hand, U.S. Pat. No. 4,081,582 employs a similar quantity of organic fibers which are fused by preheating to provide bonding for the overall fibrous material disclosed therein. U.S. Pat. No. 3,601,081, discloses the bonding of felt-like materials where organic fibers are added and then heated and cooled to provide the required bonding.
Several prior art patents disclose composite nonwoven insulation materials formed by superimposing or layering in a preselected orientation loose batts of fibers. The layered batts are sent through a needle loom, such as disclosed in U.S. Pat. No. 2,958,113, that includes a pair of vertically reciprocating needle boards containing an array of barbed needles. With the loose layers stationarily positioned between the boards, the boards are reciprocated so that the needles penetrate the layers. In this manner, the fibers of the outer layers are advanced downwardly and upwardly in the direction of the movement of the needles toward the center layer to entangle the fibers of the various layers and thereby mechanically interlock the layers. The size and the number of needles on the board, as well as the number of punching operations per square inch of the layered material, determine the density and thickness of the composite material.
For example, the nonwoven material formed by the process disclosed in U.S. Pat. No. 2,908,064 includes loose batts of synthetic organic fibrous material which, after needle punching, can also be heated to a suitable temperature to retract the fibers and increase the overall density of the composite material. Meanwhile, U.S. Pat. No. 3,317,335 employs a large quantity of organic fibers which are needle punched into a mat and then heat shrunk to insure proper bonding. Although employing a primary mat of glass fibers which are "connected" by needle punched organic fibers, U.S. Pat. No. 3,608,166 teaches preheating the organic fibers and adding a coating to the glass fibers to facilitate the needling.
Lastly, U.S. Pat. No. 3,975,565 discloses a fibrous structure that includes a plurality of interlayered inorganic fiber mats and organic fiber webs which are held together by needle punching the organic fibers from the outer web into the inorganic mat. The multi-layers are needle punched from both the top and the bottom. As a result the layers are mechanically interlocked. The preferred arrangement is to sandwich the inorganic fiber mat between organic fiber webs. The organic fiber webs are preferably fabricated of natural or synthetic fibers, for example nylon or polyester. It is further disclosed that the inorganic fiber mat can include mats of glass fibers, mineral and clay wool fibers, alumino-silicate fibers, silica fibers, and polycrystalline fibers, such as zirconia or alumina.
The above-mentioned prior art patents disclose various methods of employing organic fibers to form organic and inorganic fiber insulation mats which may be sufficiently strong and flexible to facilitate handling prior to installation. However, it should be recognized that organic fibers are subject to disintegration when exposed to elevated temperatures. In fact, for some high temperature applications, it appears that the composite insulation materials taught hereinabove would contain too high a concentration of organic fibers. As a result, destruction of the organic fibers could sufficiently reduce the mechanical strength and integrity of the insulation material to make it unsatisfactory for such use.
Although U.S. Pat. No. 3,338,777 discloses a fibrous mat which, in one embodiment, includes no organic binder, the inorganic strands must initially be crimped and relatively moved with respect to one another in all directions to insure adequate distortion. Assuming such strands could be obtained, they would appear relatively expensive to provide. In any case, the crimped inorganic strands would then be sent to a cutter machine, a garnett machine, a lapping machine, and a needle loom. Although the completed web material might be sufficiently strong and capable of withstanding higher temperatures, there is no disclosure of the resulting flexibility which can be important during handling and installation and has heretofore been made possible by the inclusion of organic fibers within the mats.
Therefore, there is a need in the formation of nonwoven insulating materials containing primary inorganic fibrous material, such as glass fibers and rock wool, to utilize a minimum amount of organic fibers for bonding together the inorganic fibers in an initial form to provide the desired mechanical strength and flexibility needed for handling prior to and during installation. However, the amount of organic fibers present should be such that, if the organic fibers are exposed to elevated temperatures after installation, their disintegration will not materially affect the mechanical strength and integrity of the composite insulating material.